The preferential displacement of silicon-nitrogen bonds in a compound that additionally contains silicon-hydrogen bonds is unknown in the art. What is well known is the preferential displacement of silicon-chlorine bonds in a compound containing silicon-hydrogen bonds. This latter reaction is characterized by side reactions and massive amounts of hydrochloric acid by product. To overcome these problems a new process is needed.
The insertion of carbon dioxide into the silicon nitrogen bond to give silyl carbamates was initially studied in the early sixties. Originally it was proposed that the insertion was effected by the displacement of the amine by a carbamic acid derivative rather than a direct two atom insertion. However, it has been noted that trisilylamine and methyldisilylamine do not appear to react with carbon dioxide. Surprisingly, the present process differs from the large body of published work in that one to three amino groups may be displaced from silicon and the displacement occurs from a silane bearing a silicon hydrogen bond and that the silicon hydrogen bond is not attacked. Thus in this catalyzed process, the initial formation of a silylcarbamate linkage seems most reasonable. The silyl carbamate may be formed either by a direct two atom insertion by carbon dioxide or by the interaction of the silyl amine with a carbamate derivative. The greater reactivity of the silyl carbamate linkage versus a silyl amine linkage toward nucleophilic displacement by an alcohol leading to the formation of alkoxysilanes was noted in U.S. Pat. No. 3,792,073; U.S. Pat. No. 3,816,359; and U.S. Pat. No. 3,906,018.
In 1975 the reaction of N,O-bis-(trimethylsilyl)carbamate with alcohols, phenols and carboxylic acids was reported to lead to the formation of trimethylalkoxy (and acetoxy) silanes, carbon dioxide and ammonia. (L. Berkofer and P. Sommer, J. Organometal Chem., 99 (1975) Cl.). While the literature thus far cited is related to the process of this invention, the use of reactions of silyl carbamate linkages to carry out nucleophilic substitution reactions with alcohols with the retention of the labile silicon hydrogen linkage went unrecognized. In addition the catalyzed reactions described herein have a significant advantage over the currently taught and practiced art. For example, the commonly used method for the preparation of trialkoxysilane suffers from several disadvantages that can be circumvented by this invention. The current art is characterized by the following: (a) solvent is sometimes employed, (b) the reaction time is relatively long in order to minimize formation of tetra-alkoxysilanes and (c) hydrochloric acid is produced.
As the process is currently understood it appears to offer a new, convenient and high yield synthesis of alkoxysilanes and trialkoxysilanes in particular. It seems most likely that the process involves intermediate silyl carbamate linkages which appear to be enormously reactive toward displacement by alcohols when compared to silyl amine linkages or the silicon hydrogen bond.
The catalyzed process described here is clearly superior for the preparation of trialkoxysilanes in that it does not require solvent, it involves short reaction times and moderate temperatures, the displaced amine is much less corrosive than hydrogen chloride, and proceeds with remarkable and unexpected selectivity for the formation of trialkoxysilanes.